A wearable device may be worn by a user. It may be desirable to authenticate such a wearable device, while an authorized user wears the device. Such authentication may be useful, for example, to positively identify that the user wearing the wearable device is an authorized user, while providing a service to the user. It may also be desirable to selectively de-authenticate the wearable device, to deter misuse of the wearable device by unauthorized users.